The present invention relates to a modified cyanate ester group curable resin composition, and varnish, prepreg, metal clad laminated board, film, printed circuit board, and a multilayered circuit board using the same.
A large volume of data is required to be processed with a high speed in a highly information-oriented society, and consequently, the frequency of signals used in computers and information terminals has become higher and higher in recent years. However, since an electric signal has a property that its transmission loss becomes larger as the frequency becomes higher, developing a low-loss printed circuit board capable of coping with such high frequencies has become an important requirement of the industry.
The transmission loss of a printed circuit board comprises a conductor loss depending on the shape of the circuit (conductor), a skin-effect resistance, a characteristic impedance, as well as the like, as well as a dielectric loss depending on the dielectric properties of the insulating layer (dielectric) around the circuit. The dielectric loss is dominant in the overall electric loss in a high frequency circuit. Therefore, in order to reduce the transmission loss of the high frequency circuit, it is apparent that reducing the dielectric constant and the dissipation factor (tan .delta.) of the printed circuit board (particularly, the insulating resin) is necessary. For example, in the field of mobile communication equipment dealing with high frequency signals, printed boards having a low dissapation factor are strongly required in order to reduce the transmission loss in a quasi-microwave band (1 to 3 GHz) as the frequency of signals is increased.
On the other hand, in the field of electronic information equipment, such as computers, the development of high speed microprocessors having an operating frequency exceeding 200 MHz and an increase of signal frequency has been advancing rapidly in order to handle increasing larger volume of information in a short time. In the equipment using such high speed pulse signals, the signal delay time on the printed circuit board becomes a problem. Since the signal delay time on the printed circuit board becomes longer in proportion to the square root of a specific dielectric constant .epsilon. r of the insulator around the circuit, resins having a low dielectric constant are required for circuit boards used in a computer and the like.
Regarding resin compositions for improving the high frequency property of the printed circuit board and which are capable of coping with the trend toward the use of increasingly higher frequency signals, as described above, a method using a Cyanate ester/epoxy resin composition has been disclosed in JP-B-46-41112 (1971), and a method using a bismaleimide/Cyanate ester/epoxy resin composition has been disclosed in JP B-52-31279 (1977), as a composition using a Cyanate ester resin having the lowest dielectric constant among thermosetting resins.
As a method of improving the high frequency property using a thermoplastic resin, methods of using polyphenylene ether group resin compositions having a desirable dielectric property among heat-resistant thermoplastic resins, such as a resin composition composed of a polyphenylene ether resin (PPO or PPE) and a cross linking polymer/monomer has been disclosed in JP-B-5-77705 (1993), and a resin composition composed of a polyphenylene ether having a specific curable functional group and a cross linking monomer has been disclosed in JP-B-6-92533 (1994).
Further, as a means of improving the high frequency property using a resin composition composed of a cyanate ester resin having a low dielectric constant and a polyphenylene ether resin having a desirable dielectric property, a method of using a resin composition composed of a cyanate ester/bismaleimide and a polyphenylene ether resin has been disclosed in JP-B-63-33506 (1988), and a method of using a resin composition composed of a phenol modified resin/cyanate ester reactant and polyphenylene ether resin has been disclosed in JP-A-5-311071 (1993). Furthermore, a resin composition prepared by kneading a polyphenylene ether resin and a cyanate ester resin has been disclosed in JP-B-61-18937 (1986) as a heat resistant molding material having a desirable frequency characteristics.
The methods disclosed in JP-B-46-41112 (1971) and JP-B-52-31279 (1977), respectively, had a problem in that the high frequency property was insufficient though the dielectric constants were slightly lowered, because the resin compositions contained thermosetting resins other than a cyanate ester resin.
The methods disclosed in JP-B-5-77705 (1993) and JP-B-6-92533 (1994) also had a problem in that the resin compositions were high in molten viscosity and lacked in resin flow, though their dielectric constants were somewhat improved, because the main component of the resin compositions was a polyphenylene ether resin which was essentially a thermoplastic resin. Therefore, the resin compositions required a high temperature and a high pressure for pressurized molding of the laminated board, and were unsuitable for forming a multilayer printed circuit board, which a required to fill a very small space in a circuit pattern, because of insufficient moldability.
The methods disclosed in JP-B-63-33506 (1988) and JP-A-5-311071 (1993) had a problem in that the high frequency property was still insufficient, though the dielectric constants were somewhat improved, because the thermosetting resin used together with the polyphenylene ether resin was a bismaleimide/cyanate ester resin or a phenol modified resin/cyanate ester reactant. When the quantity of polyphenylene ether resin was increased, the resin composition became high in molten viscosity and lacked in fluidity, thereby to decrease the moldability, similar to the case of the polyphenylene group resin composition described above.
The resin composition prepared by kneading polyphenylene ether resin and cyanate ester resin disclosed in JP-B-61-18937 (1986) had a desirable dielectric property and a relatively preferable moldability, because the molten viscosity was lowered by being modified by cyanate ester resin. However, when cyanate ester was singly used as a curing composition, the problem that the dielectric property of the cured resin had a high dissapation factor, while the dielectric constant was relatively low, still remained. Further, when the quantity of cyanate ester was decreased (the quantity of polyphenylene ether resin was increased) in order to lower the dissapation factor, the problem that the molten viscosity of the resin composition was increased to make the fluidity insufficient still remained, and the moldability was decreased, similar to the case of the polyphenylene group resin composition described above.